Devices for transmitting torque between two rotatable shafts are known from numerous patent publications, one good example being U.S. Pat. No. 3,987,689, where the two shafts are the output shafts of a vehicle differential mechanism. The device may accordingly in this case be called a differential brake. There may, however, be other instances when it is desired to counteract a certain rotational speed differential between two shafts, a typical example being the two shafts to the driven front and rear axle, respectively, of a four-wheel-drive vehicle.
Whereas the above-mentioned patent publication shows a "speed-sensitive differential mechanism" and the present invention is applied to the more general case with two axially aligned shafts, the same general considerations apply.
Although U.S. Pat. No. 3,987,689 has been taken as an example of the prior art above, there are many other publications suggesting solutions to the problem of decreasing undesired rotational speed differential between two shafts, such as EP-Patent-0 350 044, EP-B-0 368 893, U.S. Pat. No. 4,012,968, U.S. Pat. No. 3,488,980 and U.S. Pat. No. 5,087,228.
In a copending patent application from the applicant (SE 9202200-3) a device as defined above is shown. Here the hydraulic clutch piston as well as the entire hydraulic pump arrangement are placed in the stationary housing, which gives considerable advantages. However, a possible drawback with the device shown therein is that the hydraulic flow and thus the pressure to the clutch piston from the hydraulic pump arrangement fluctuates during each operation cycle or differential rotation between the two shaft members.
The main object of the present invention is accordingly to obviate the drawback in a device with the hydraulic clutch piston and the hydraulic pump arrangement in the stationary housing that the pressure on the clutch piston fluctuates during one differential rotation between the two shaft members.
A similar object has been attained in a way which is described in GB-A-2 229 255 in a "hydraulic transmission device".